As an example, suppose that transcription factors influence transcription only when they are bound to their target sites. In this case the cell does not have direct access to the concentration \(g\)  but only to the binding site occupancy averaged over time; there could also be multiple binding sites. Abstractly, the concentration \(g\)  is being mapped into some other variable, and we can call this variable \(C\) , without worrying about details. The crucial consequence of noise in the binding events is that \(C\)  can provide only a limited amount of information about the concentration \(g\)  [1]}. Different molecular mechanisms generate different mappings \(g \rightarrow C\) , but in all mechanisms the low concentrations of the relevant molecules limit the information that is transmitted, \(I(C; g)\) .
